Abstract
Continuous tracking is a newly developed technique that measures the correlation between a randomly changing stimulus property (usually 2-D position) and the response of participants tracking the object. This technique can in principle by generalised to measure any dynamic aspect of a stimulus, to provide useful information not only about sensitivity, but also dynamics and contextual effects. Here we apply it to motion and numerosity.
Participants tracked the direction of motion of 1-D noise moving randomly over a randomly moving background, target and background following independent motion trajectories. Observer responses correlated positively with the target motion, and negatively with the background motion, demonstrating and quantifying surround inhibition of motion.
Separately, participants tracked on a number-line the perceived numerosity of a cloud of dots. Some dot-pairs were connected by lines, producing an illusory reduction of the apparent numerosity of the dot clouds: both the number of dots and the proportion connected by lines varied over time, following independent random walks. The tracking correlations showed that grouping dots by connecting lines caused a robust underestimation of numerosity. The tracking response to the illusion created by connection was about 150 ms slower than to the physical numerosity, suggesting that this time was utilised in processing the grouping effect.
Finally, we developed an ideal observer model that closely models human results, providing a generalized framework for modelling the effects on tracking data, and to study the divergence of human participants from ideal behavior.